lab colloid chemistry & turbidity
TRANSCRIPT
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EVEN 3321
Fall 2011EVEN 3321
Objectives
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1. To understand what colloids are & why they are important in environmental engineering.
2. To understand the electric double layer theory ofcolloidal surface charge.
3. To understand thedifference between electrostatic repulsive forces & van der Waals’ attractive forces between colloidal particles.
4. To understand theelectrokinetic propertiesof colloids(e.g., zeta potential & electrophoretic mobility).
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Objectives (cont.)
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5. To understand how colloids can be destabilized & coagulated (e.g., by increasing ionic strength or adjusting pH).
6. To understand the meaning of “pointof zero charge”and pHpzc.
7. To understand the causes & significance of turbidityin water supplies.
What are “colloids”?
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� Colloids = particlesof 1-1000 nm (1nm = 10-9m).
� Can exist as dispersions in solids, liquid, orair.
� Sols = solid colloids in liquid (this lab)
� Emulsions = liquid colloids in liquid
� Foams = gas colloids in liquid
� Smokes = solid colloids in gas
� Fogs = liquid colloids in gas
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General properties of colloids
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� Stability = resistance of colloid to removal by settling or filtration
� Stabilityof colloids in solution affected by:
� Particle size
� Particle surface charge
� pH, ionic strength, & organic content of water
� Surface area to mass ratio is very high then surfacephenomena predominate.
Electrical properties of colloids
� Colloidal particlesgenerally have surface charge.
� Can be positiveor negative.
� Like charges repel, preventing colloids from agglomerating(coagulating) into larger particles.
� Thus, colloidal stability largely due to surface charge.
• When charged colloids are placed in electric field they migrate towards pole of opposite charge.
� Particles with a greater surface charge exhibit a higherelectrophoretic mobility (higher velocity in electric field).
� This allows colloid surface charge to be quantified.
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Brownian Movement & Tyndall
Effect
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� Brownian movement:
� Colloidal particles are constantly in motion due tocollisions with molecules in solution.
� Tyndall effect:
� A beam of light passing through a colloidal dispersion will be ref lected
� Ref lected light can be observed at a right angle to beam oflight.
� True solutions & coarse suspensionsdo not produce thisphenomenon.
� Thus Tyndall effect used to prove presence of colloids.
Adsorption by colloids
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� Colloids have great adsorption capacity due to very large surface area.
� Colloidswill preferentiallyadsorb positive or negativecharged ions giving the colloid a net surface charge.
� Surface charge provides stability (preventing agglomerationand coagulation) for colloids in solution.
� Transportof environmentallysignificant contaminants(e.g., metals) is facilitated by adsorption to colloids.
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Colloidal solids in liquids
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� Colloidal dispersions of solids in liquids are “sols”.
� Hydrophobiccolloids all have a surface charge.� easier to remove than hydrophiliccolloids.
� The surface charge (or “primary charge”) depends on:� Character of thecolloid.
� ionic characteristics of solution, including pH.
� Surface charges tend to be negative.
� However, low pHs tend to result in more positivesurface charge.
� Colloids in solution do not settle due to gravity.
Electric “double layer” theory
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� Sol as a whole must be neutral
� Primarycharge – charged groupswithin particle surface+ adsorptionof layerof ions at surface.
� (see next slide)
� Primarycharge must be balanced by counter ions nearthe surface & in solution.
� (see next slide)
� Result is an electric double layer:
� Fixed or Stern layer of counter ions
� Diffuse layer of a mixture of charged ions.
� (see next slide)
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Electrical double layer of negatively charged colloid
Surface charge
(or primary charge)
BACK
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Electric double layer theory (cont.)
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� Fixed & diffuse layers are separated by a shear surface.
� The fixed layerwill movewith colloid if it is subjected to anelectric field.
� (see next slide)
� Counter-ions in fixed layerare attracted electrostatically.
� However, counter ions can diffuse away from fixed layerdue to Brownian motion.
� (see next slide)
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Electrical double layer of negatively charged colloid
Surface charge
(or primary charge)
BACK
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Electric double layer theory (cont.)
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� Competing forces of electrical attraction & diffusion(due to concentration difference) spread charge overthe electrical double layer.� Conc. of counter ions is greatest at surface & decreases
with distance from surface.� (see next slide)
� The primarycharge produces an electric potentialbetween the surface & the solution.� Theelectric potential is greatest at the surface &
decreases with distance from the surface.� (see next slide)
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Electrical double layer of negatively charged colloid
Surface charge
(or primary charge)
BACK
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Electric double layer theory (cont.)
� As two negatively-charged colloids come closer (r smaller), the electrostatic repulsive force between thetwo primary charges (same sign) increases (Frepel
� (see next slide)
1/r2).
� The electrostatic repulsive forces are counteracted by anintermolecularattractive force.
� Theattractive “van der Waals’ force” decreases rapidlywith distance from surface (-Fattract
� (See next slide)
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1/r6).
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Effect separating distance between colloidson forces of interaction between them
BACK
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Electric double layer theory (cont.)� Weakvan derWaals’ intermolecularattractive forces arisewhen
moleculesare in veryclose proximity (a few angstroms – 10-10m).� “Synchronized” induced dipoles result in weak electrical attraction
between molecules:
• If two colloids can be brought sufficiently close so van der Waals’ forces are greater than electrostatic repulsive forces, the two colloids will coagulate together.
o (see next slide)
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Effect separating distance between colloidson forces of interaction between them
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Electric double layer theory (cont.)
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� Todestabilize & coagulate colloidal particles:
� Need to provide kinetic energy (by stirring) to overcome theenergy barrier, or
� Reduce the energy barrier by some means.� (see next slide)
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Effect separating distance between colloidson forces of interaction between them
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Electric double layer theory (cont.)
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� One way to decrease energy barrier is to increase ionconcentration in solution (high ionic strength).
� This decreases thickness of the electric double layer.� (see next slide)
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Effect of ionic strength on energy barrier thatprevents coagulation of colloids
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Colloid electrokinetic properties� To predict conditions that will destabilize colloids, it is
useful to estimate their surface charge.
� The surface charge of colloids can be estimated by experimentally measuring their electrophoretic mobility (essentially theirvelocity in an applied electric field).
+ E -
V
v
colloid with negative surface charge
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Instrument for measuring electrophoretic mobility &
zeta potential
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Colloid electrokinetic properties
(cont.)
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� A colloid’selectrophoretic mobility is directly related to itszeta potential.
� (see next slide)
� A colloid’s surface charge (coulombs/m2) can be estimatedfrom it’s zeta potential.
� Zeta potential measurementsare used to characterize effectiveness of lowering energy barrier between colloids:
� by adding electrolyte
� by adjusting pH
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Electrical double layer of negatively charged colloid
Surface charge
(or primary charge)
BACK
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Effect of ionic strength on energy barrier thatprevents coagulation of colloids
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“Point of zero charge” & pHpzc
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� The pzc “point of zero charge” or “isoelectric point”occurs when the colloid surface charge is zero.
� Surface charge changes with pH:
� The pH at point of zero charge is called pHpzc.
� Colloidsare generally least stable (i.e., tend tocoagulate readily) at pHpzc.
� (see next slide)
Effect of pH on surface charge of clay, iron, &
aluminum colloids
pHpzc30
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Colloid destabilization &
coagulation
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� Destabilizing colloids allows them to coagulate into largerparticles that can be removed by settling.
� Four basic mechanisms for coagulating colloids:
� Electrical double layer compression.
� Charge neutralization.
� Entrapment in precipitate.
� Interparticle bridging.
(1) Electrical double layer
compression� High electrolyteconcentration:
increases concentration of ions in double layer
decreases double layer thickness
decreases energy barrier
increases colloidal coagulation
� Ions with higher charge (e.g., Al3+) are moreeffectivethan ions with lowercharge (e.g., Na+).
� (see next slide)
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Effect of ionic strength on energy barrier thatprevents coagulation of colloids
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“alum”
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(2) Charge neutralization
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� Addition of hydrophobic moleculesof oppositechargethatcanadsorbonto the colloids:
neutralizescolloidal surfacecharge
reduceselectrostaticrepulsive forces increasing colloidalcoagulation
� Dodecylammonium (C12H25NH3+) is an example.
� Addition of Al(III) & Fe(III) salts producesproducevarioushydroxidecomplexes [e.g., Al13(OH)34 ]:
Positively-charged complexesadsorbto colloids.
Results in charge neutralization.
� Overdosing can result in chargereversal & formationof stablepositively-charged particles.
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(3) Entrapment in precipitate� Addition of high doses of Al(III) & Fe(III) salts rapidly
forms hydroxide precipitates [e.g., Al(OH)3(s) & Fe(OH)3(s)]:
Colloids become enmeshed in settling sweep f loc.
Lowest solubility for Al(OH)3(s) occurs near neutralpH.
Thus coagulation best carried out at neutral pH.
(see next slide)
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Solubility of Al(OH)3(s) vs. pH
Lines are for five hydroxide complexes: AlOH2+,Al(OH)2+, Al(OH)3(aq), Al(OH)4-, & Al(OH)45+
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(4) Interparticle bridging� Addition of long-chain polymers (polyelectrolytes) can
form bridges between colloidal particles:
Numerouscommercial polyelectrolytesavailable.
Interparticle bridging between colloids using long-chain charged polymers
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Turbidity
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� Turbidity is caused by suspended matter that interfereswith passage of light.
� Suspended mattercan range from colloidal to coarsedispersions & can be either organic or inorganic.
� Colloidal rock particles
� Topsoil, clays, and silt
� Domestic & industrial wastewater
� Street runoff
� Bacteria, algae, & other microorganisms
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Environmental significance of turbidity
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� Aesthetics
� Filterability
� Operation of rapid sand filters requires prior removal ofturbidity by chemical coagulation.
� Increasing turbidity increases difficulty & cost of filtering water supplies.
� Disinfection
� Usually uses chlorine, ozone, ClO2, or UV radiation.
� Disinfecting agents must be in contact with theorganism.
� In turbid water supplies, microorganismscan be encased in particles & thus protected from disinfection.
Turbidity measurement, units, & standards
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� Turbidity is measured using nephelometry:� Light source illuminates water sample
� Photoelectric detectors measure intensity of lightscattered at right angles.
� Turbidity measurementsare reported innephelometric turbidity units (NTU).
� EPA set more stringent turbidity standards fordrinking waters in 2002.� Turbidity must never exceed 1 NTU.
� Turbidity must not exceed 0.3 NTU in 95% of dailysamples in any one month.
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Application of turbidity data
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� Turbidity measurements can help determine the following for water supply treatment plants:
� Whether a raw water supply requires chemical coagulation prior to sand filtration.
� Optimal coagulant [e.g., Al(III) or Fe(III) salts].
� Coagulantdose required.
� Sand filter effectiveness.
� Conformity with regulatory standards.